How to scale objects along the length of a curve and keep them all facing the same way
As far as I know you will either be able to scale them progressively or orient them correctly at once, not both.

Using the arch as duplifaces in an arrayed plane you get correct orientation always facing one axis but no scaling.



You can then make duplicates real and clear any erroneous rotations introduced by the curve modifier.



Then scale them by hand or using proportional edit as desired.



Otherwise use the the array directly on the arches. It will not orient them correctly, but you can control scaling point buy point through the curve radius property.